Process for separating hydrocarbons



Feb. 27, 1951 A. E. HIRSCHLER 2,543,285

PROCESS FOR SEPARATING HYDROCARBONS Filed Dec. 26, 1947 o g a :3 (D 2 2-I D F8 0: a D o o in 0%A 56%A 0093A Q E5 0: 5 3 8 a 2:

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i I i/M 093A 56% IOO%A |OO%B 50 0%5 I INVENTOR.

ALFRED E. HERSCHLER BY ATTORNEYS Patented Feb. 27, 1951 '"PBLQCESS 'FORSEBARATING HYDROQABBQNS Alfred Hirschler, Drexelfliill,,l?a., assignorto Sun Oil Company, Philadelphia, Pa a,-corp0ra-.-

tionof New Jersey Application-December 26, 1947, Serial'No.'794,033

6 Claims. 1

This invention relates to the separation of hydrocarbons by adsorptionand particularly to the separation of hydrocarbons having the samedegree of saturation. More specifically, the invention relates to aprocessfer separating such hydrocarbons involving at least twoadsorption steps conducted at different operating temperaures.

Methods are ,known for separating hydrocarbons by .treating a mixture ofthe same with a granular adsorbent material such as silica gel. oractivated carbon to remove one or more of the hydrocarbons by selectiveadsorption. This p o e u ha b n employ mainly or sep rating hydrocarbonsaccording to chemical type where the different types of hydrocarbonsconcerned have greatly difierentadsorptiveproper ties. Thus, adsorptionmethods have been utilized to remove unsaturated hydrocarbons such asolefins or aromatics from saturated hydrocarbons such as paraffins ornaphthenes. As a g n al ule, the d crptivity of hydrocarbons increasesas the unsaturation vor number of double bonds per molecule increases,so that an. unsaturated hydrocarbon may readily be selectively adsorbedfrom a mixture containing the, 1same together with a more saturatedhydrocar- It has generally been thought that hydrocarbons having thesame degree of saturationwould not exhibit anyfiubstantial difference inadsorbability, especially when the hydrocarbons are not greatlydifierent in molecular weight. For example, ithasbeen generallyconsidered that two paraflin hydrocarbons, or two naphthenes, would haveso nearly the same adsorbabilities as not to be separableby adsorptionand especially so when the hydrocarbons are isomers or adjacenthomologues. Likewise, the adsorption method has been presumed to beineffective to separate two olefins having thesame number'of doublebonds per molecule, or two aromatics having the same number of aromaticrings ordouble bonds per molecule.

More recently, however, I have :found that hydrocarbons which have :thesame degree of saturation-may be separated byselectiveadsorpl m cy nadso nt s as s lica. eel or activated carbon. This istrue even ofhydro.- carbons that are very closely related such as isomers oradjacent homologueaand aseparation of such hydrocarbonsgenerallymayreadily be accomplished in this manner. Methods of separating varioustypes of hydrocarbons of the same degree of saturation are disclosed andclaimed in my e-p n a pli ations ollows: :Serial Nos. 6.43.762, ,now'abandoned, 643,763, now Patent No. 2,448,488, and 643,754, new PatentNo.'2,,464,931, filed January 26, .1946;, serial. No. 660.97 ow a doned.fil A il 1946; ,Serial Nos. 672,683, now abandoned, 672,684, nowabandoned, 672,685., new Patent No. 2..48.Q,2 2 and 67 86 e a en Ne2.532.5 8 filed May 10, 1947; and Serial No. 747,277,, now Patent N 2.58.236 ed M y Ql" 9 7.

With agiven adsorbent and at a given operat: in temp at r he e r icnbehavior of h dr arb n pairs i lW fi-h h ce p n shave thesame degree ofsaturation may .be classified as oftwo general types. With somehydrocarbon pairs, one of the members will be selectively adsorbable atall concentrations. With ether pairs of such hydrocarbons, one of themembers will be selectively adsorbable within a certain concentrationrange While the other member will be the more adsorbable c mponent atall other compositions of the mixture.

Thesetypesofbehavior can best be'illustrated b mea s of ads rptio otherch s are shown in Figures 1 and 2 oi the accompanying drawings. :As iswell known, the adsorption isotherms show the relationship between,composition of the "mixture at equilibrium with ,a given amount ofadsorbent ,and the amountot component adsorbed. (The latter :value isthe apparent amount as customarily calculated from the change .incompositionof the mixture beforeandafter adsorption assuming no adsorp-.tion of the other component.)

Figure 1 represents the type. of behavior where, at a given temperature,oneof the hydrocarbons is preferentially adsorbed at all concentrations.From a mixtureof .the hydrocarbons, .A and B, hydrocarbon A isselectively adsorbed regardless of its proportion :in the mixture. Thetype of curve shown in Figure 1 is generally referred to as a U-typeadsorption isotherm.

Fi ur 2 l tr s t oth r t p of s s em in which, at a given temperature,one of the components is selectively removable within ,a e t nconcentration ran whereasat all other concentrations the otherhydrocarbon is the, preferentially adsorbable component. ,At the pointwhere the adsorption isotherm curve crossesthe horizontal line(correspondi g to the composition represented by point M?)., there isnodifierence in adsorbability and neithercomponent is selectivelyremovable. Thus, com.- p n A an be selectiv ly adsorbed from any mixtureotA and B which containsbetween 0% 3 A and the percentage correspondingto M, while component B can be preferentially removed when thepercentage of A is above that corresponding to M. At composition M, noseparation can be obtained.

It is apparent that when the starting mixture of the type illustrated byFigure 2, the amount of separation obtainable at a given operatingtemperature will be limited. While it will be possible to obtain afraction containing one of the components in relatively pure form, themaximum possible concentration for the other component will be thatcorresponding to point M. When a charge material of this type is passedthrough a bed of adsorbent, the more strongly adsorbed component will bepreferentially retained by the adsorbent and the other component can beobtained in relatively high concentration in the first portion offiltrate. However, upon displacing all of the charge hydrocarbons fromthe bed as by means of a suitable desorbing agent, the composition ofthe latter fractions may approach that corresponding to point M butcannot exceed the concentration of the more adsorbable component at M.This applies regardless of'whether the proportion of A to B in thestarting mixture is such that A is selectively adsorbed or such that Bis the more adsorbable component.

The present invention is concerned with a process for separatinghydrocarbons which exhibit an S-tYpe adsorption isotherm, as illustratedin Figure 2. Lhave discovered that with mixtures of this type theproportion of components at which the hydrocarbons have the sameadsorbability varies with the operating temperature. In other words, theposition of point M in Figure 2 changes as the temperature changes, sothat a mixture which is not separable at one temperature can beseparated by means of the same adsorbent at a substantially differenttemperature. Briefly, the process comprises treating a mix ture of suchhydrocarbons in liquid phase with a selective adsorbent in a first stepand obtaining a fraction in which the hydrocar bons are present in aproportion approaching that at which their adsorbabilities are the same,in other words in a proportion approximating that, or not greatlydiflerent from that, at point fM. This fraction is then treated in asecond step with adsorbent of the same type but at a substantiallydifferent temperature from that of the first treatment to efiect furtherseparation of the hydrocarbons.

The eifect of temperature in changing the point where the componentshave identical adsorbabilities applies for any mixtures of hydrocarbonshaving the same degree of saturation which mixture exhibits an S-typeadsorption isotherm. Such mixtures are included within each of thefollowing types of hydrocarbon combinations: n-parafiin-n-parafiin;isoparaffin-isoparaffin; n-paraffin-isoparafiln; n-paramn-naphthenei'soparafiin-naphthene; naphthene-naphthene; mono-olefin-mono-olefin;diolefin-diolefin; and aromatic-aromatic. The invention may be appliedin the separation of any of such mixtures which have an $-typeadsorption isotherm.

The preferred adsorbents for practicing the invention are silica gel andactivated carbon. The invention is applicable, however, where otheradsorbents are used provided the adsorbent chosen has a sufficientlyhigh adsorptive capacity for the charge hydrocarbons and an S-typeadsorption isotherm obtains at the operating temperature in the firststep. Among the other adsorbents which may be useful, dependent uponwhat the particular hydrocarbons are in the charge, are activatedaluminas such as bauxite, activated magnesia and various activatedclays. As a general rule, however, silica gel or activated carbon willeffect a much better separation than other commercially availableadsorbents.

Any suitable operating temperatures may be employed in the twoadsorption steps but the temperature in the second step should besufficiently difierent from that in the first to cause a substantialshift in the composition at which there is no selective adsorption.

As a general rule, the difference between the two temperatures should beat least 15 C. and preferably should be more than 30 C. The temperaturein the second step may be either higher or lower than in the first stepprovided a suificient temperature diiference is maintained. It isgenerally desirable to avoid the use of high temperatures, since theadsorptive capacity of the adsorbent will decrease as the temperatureincreases. For this reason, it is advantageous to provide refrigeratingmeans so that as low a temperature as is feasible may be employed in oneof the two steps.

In practicing the process, each adsorption step is carried outpreferably by filtering the charge through a bed of the adsorbent, asfor example by percolatin the charge through a column containing arelatively large amount of the adsorbent. The second step may be carriedout in a different column from that used in the first step or in thesame column after the adsorbent has been regenerated. In each step afterall of the charge has passed into the adsorbent, it may be followed by adesorbing agent comprising a liquid more strongly adsorbable than theadsorbate to effect its displacement. Polar organic liquids, such asalcohol or acetone, are particularly suitable for desorbing silica gel.Aryl compounds, such as benzene, toluene, xylene, phenol or the like,may be used to desorb activated carbon. Aromatic hydrocarbons may alsobe used to desorb silica gel when the adsorbate comprises saturatedhydrocarbons, olefins, or diolefins. The desorbing agent should be soselected with respect to boiling point that it will be readily separablefrom the desorbed hydrocarbons by distillation. In each treating stepthe efilux from the column may be collected in separate fractions asdesired in order to segregate those portions having the desired purity.In case the desired purity of the less adsorbable component is notattained by one percolation in either the first or second step, thefiltrate fractions may be retreated with fresh adsorbent at the same ora difierent temperature toefiect further purification.

By way of example, the treatment of a mixture of 2,2,4-trimethylpentaneand methylcyclohexane is illustrative. It was found that this pair ofhydrocarbons with a silica gel adsorbent gave an S-shaped adsorptionisotherm at 10 C., with the point of no selectivity occurring at 19% byvolume of 2,2,4-trimethylpentane. At the same temperature but below thispercentage the 2,2,4-compound is selectively adsorbed, while above thispercentage the methylcyclohexane is selectively adsorbed. However, itwas found that at a temperature of 60 C. the point of no selectivityoccurred at 61% by volume of 2,2,4- trimethylpentane. Thus, if astarting mixture of these two compounds containing substantially lessthan 19% 2,2,4-trimethylpentane is percolated through a bed of silicagel at 10 C., the

'2 ,2,4-trimethylpentane will be selectively sad- -sorbed and the firstfraction-of filtrate willlbe enriched with respect to thmethylcyclohexane.

Upon displacing the adsorbate from the :gel,

starting with :a imixture of :these compounds containing :(say) '70%.2,2,4-trimeth3rlpentane,.it would be possible theoretically to obtain.each component :in relatively :pure form merely by In every case thechange in temperature shown effected a substantial shift in the point atwhich the two components have the same degree of adsorbability. It is tobe understood that the particular systems listed are merelyillustrative, having been selected at random, and that there are a vastnumber of other systems to which the invention is applicable.

In some cases a change in the operating temperature may cause theadsorption isotherm to change from S-type (as in Figure 2) to U-type (asin Figure 1). This is illustrated by the system, 2,2,4-trimethylpentaneand methylcyclohexane with activated carbon as the adsorbent. At 60 C.,the point of zero selectivity occurs at a proportion of 35% by volume of2,2,4-trimethylpentane, with this being selectively adsorbed at allproportions below this value While the methylcyclohexane is selectivelyadsorbed at all higher proportions. However, at 10 C., the2,2,4-trimethylpentane compound is selectively adsorbed at allproportions. In 75 one or more fractions may be obtained having treatingwith activated carbon at C. kI-Idwa composition'approaching thatat'which neither ever, in practice it has been found to be difficultcomponent is selectively 'adsorbable at 10 C. to obtain the adsorbatefractioniin high purity. (i. e. approaching 19% 2,2,4-trimethylpentane).It may well "beadvantageous, therefore, to treat These fractions maythen be treated by .percolathe mixture in accordance: with -the :presentintionthrough a bed of silicagelat 60 C. :and 10 -'vention by contactingit with activated :carbon further selectiveadsorption of the2;2,4-1triin a first .step conducted at about -.60 C. to methylpen'tanewill-be obtained. In this :second selectively sad'sor'b the'methylcyclohexane :and treatment, one or more adsorbate iractions mayyield the 2g2A- compou1idiinrelativfelyhigh:pnrity be obtained in whichtheconcentration of 2,2,4- as'filtrate. ,Tlretadsor'bate fractionsubsequently trimethylpentane approaches 61%. The latter 1'5 may-betreated-with activated carbon inasecond fractions may, if desired, thenbe treated with ste satna temperature of veabout 10 C.-to;se1ecsilicagel at 19 C. to eflect further separation, 'tively iadsorb the2,2;4-trimethylpentane and in which case the methylcyclohexane will beyield methylcyclohexane of high ,purity as filselectively adsorbed andthe 'first;portions of filtrate. trate will contain the2,2,4-trimethylpentane in 20 While the invention hasibeen-illustratedwith relatively pure form. Thus, either componentreference to certain specific hydrocarbons, it is of the startingmixture mayibe obtained inhigh by no means limited thereto but :may:ber'applied purity. to the separation :of any .pair of hydrocarbons Onthe other hand, if the starting mixture havingthesamedegree ofsaturatiomorthesame contains more than 19% 2,2,4-trimethyl'pentanenumberof double'bonds per molecule,and'which and is treated with silicagel at 10 (3., the have anES-type'adsorption-isothermat the tem-- yyclohexane will be the co p that pera'ture employed in the firstadsorption step. is selectively adsorbed and the '2,2,4-'trimethyl-Having described my imvention, What 1.01am

D e Will be Obtained in enriched form 1&5 and'desire to protect byLetters Patent is: e first portioltls 0f filtrate- Trefltment 0f the 1.In a process of separating'hydrocarbons of adsorbate fractlon at W111then cause thesame degree'of saturationby selective adsorppreferentialadsorption o the 2,2,4-compound, tion'whereinaliquid mixture of suchhydr0carpermitting the methylcyclohexane to be obtained bons treatedwith granular adsorbent. a s filtrate of high p ytemperature at whichthe hydrocarbonsexhibit Various specific examples of starting mixtures.35 an 'S-type adsorption isoth r and at m t to which the invention isapplicable with silica i thereby eparated nt a frag-( 1 'cgntainjng gelas the adsorbent ware shownin the following one of the hydrocarbons inrelatively purified which iv s c mp i i n rre p n formand anotherfraction containing itheicharge to the points of zero selectivity atdifferent temhydrocarbons. in a proportion approachingithatat peratures.40 :which theiradsorbabilitiesare zthe same,'the im- Adsorbent: silicagel tttttttti" Component A Component B 10 Oi C. 0.

Percent Per cent Per cent 2,2,4-trimethylpentane. methylcyclohexane 1961 Do n-octane 40 54 n-heptane methylcyclohexana- 5O 69 n-octaneethylcyclohoxane 30 53 2,2,5-trimethy]hei ane do 14 30 M provement whichcomprises then treating the last-named fraction in liquid phase withadsorbent of the same type at a substantially difierent temperature fromthat of the first treatment,

thereby eifecting further separation of said hydrocarbons.

2. In a process of separating hydrocarbons of the same degree ofsaturation by selective adsorption wherein a liquid mixture of suchhydrocarbons is treated with silica gel at a temperature at which thehydrocarbons exhibit an S-type adsorption isotherm and the mixture isthereby separated into a fraction containing one of the hydrocarbons inrelatively purified form and another fraction containing the chargehydrocarbons in a proportion approaching that at which theiradsorbabilities are the same, the improvement which comprises thentreating the last-named fraction in liquid hase with silica gel at asubstantially different temperature from that of the first treatment,thereby efiecting further separation of said hydrocarbons.

3. In a process of separating hydrocarbons of the same degree ofsaturation by selective adsorption wherein a liquid mixture of suchhydrocarbons is treated with activated carbon at a temperature at whichthe hydrocarbons exhibit an S-type adsorption isotherm and the mixtureis thereby separated into a fraction containing one of the hydrocarbonsin relatively purified form and another fraction containing the chargehydrocarbons in a proportion approaching that at which theiradsorbabilities are the same, the improvement which comprises thentreating the last-named fraction in liquid phase with activated carbonat a substantially different temperature from that of the firsttreatment, thereby effecting further separation of said hydrocarbons.

4. In a process of separating hydrocarbons of the same degree ofsaturation by selective adsorption wherein a liquid mixture of suchhydrocarbons is passed through a bed of granular adsorbent at atemperature at which the hydrocarbons exhibit an S-type adsorptionisotherm and the filtrate from the bed is collected in separatefractions including a fraction containing one of the hydrocarbons inrelatively purified form and another fraction containing the chargehydrocarbons in a pro-portion approaching that at which theiradsorbabilities are the same, the improvement which comprises thenpassing the last-named fraction in liquid phase through a bed of thesame type of adsorbent at a substantially different temperature fromthat of the first treatment, thereby effecting further separation ofsaid hydrocarbons.

5. In a process of separating hydrocarbons of the same degree ofsaturation by selective adsorption wherein a liquid mixture of suchhydrocarbons is passed through a bed of silica gel at a temperature atwhich the hydrocarbons exhibit an S-type adsorption isotherm and thefiltrate from the bed is collected in separate fractions including afraction containing one of the hydrocarbons in relatively purified formand another fraction containing the charge hydrocarbons in a proportionapproaching that at which their adsorbabilities are the same, theimprovement which comprises then passing the last-named fraction inliquid phase through a bed of silica gel at a substantially differenttemperature from that of the first treatment, thereby efiecting furtherseparation of said hydrocarbons.

6. In a process of separating hydrocarbons of the same degree ofsaturation by selective adsorption wherein a liquid mixture of suchhydrocarbons is passed through a bed of activated carbon at atemperature at which the hydrocarbons exhibit an S-type adsorptionisotherm and the filtrate from the bed is collected in separatefractions including a fraction containing one of the hydrocarbons inrelatively purified form and another fraction containing the chargehydrocarbons in a proportion approaching that at which theiradsorbabilities are the same, the improvement which comprises thenpassing the last-named fraction in liquid phase through a bed ofactivated carbon at a substantially different temperature from that ofthe first treatment, thereby effecting further separation of saidhydrocarbons.

ALFRED E. HIRSCHLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

Hirschler et aL, Ind. Eng. Chem, vol. 39, 1587 (1947).

Mair et al., Ind. Eng. Chem., vol. 39, 1072-81 (1947).

1. IN A PROCESS OF SEPARATING HYDROCARBONS OF THE SAME DEGREE OFSATURATION BY SELECTIVE ADSORPTION WHEREIN A LIQUID MIXTURE OF SUCHHYDROCARBONS IS TREATED WITH A GRANULAR ADSORBENT AT A TEMPERATURE ATWHICH THE HYDROCARBONS EXHIBIT AN S-TYPE ADSORPTION ISOTHERM AND THEMIXTURE IS THEREBY SEPARATED INTO A FRACTION CONTAINING ONE OF THEHYDROCARBONS IN RELATIVELY PURIFIED FORM AND ANOTHER FRACTION CONTAININGTHE CHARGE HYDROCARBONS IN A PROPORTION APPROACHING THAT AT WHICH THEIRADSORBABILITIES ARE THE SAME, THE IMPROVEMENT WHICH COMPRISES THENTREATING THE LAST-NAMED FRACTION IN LIQUID PHASE WITH ADSORBENT OF THESAME TYPE AT A SUBSTANTIALLY DIFFERENT TEMPERATURE FORM THAT OF THEFIRST TREATMENT, THEREBY EFFECTING FUTHER SEPARATION OF SAIDHYDROCARBONS.